The goal of this study is to gain mechanistic insights on the function of the Werner syndrome protein (WRN) complex in the processes that maintain telomere integrity and prevent the formation of extrachromosomal telomeric circles. WRN is a nuclear protein with helicase and exonuclease activities, whose loss-of-function mutations are associated with the premature aging and cancer prone disease Werner syndrome (WS). Genetic and biochemical evidence implicate WRN in telomere metabolism and suggest that abnormal telomere length homeostasis contributes to the pathology of WS. Our biochemical studies have shown that WRN operates as a functional unit with the Ku70/80 heterodimer and have further demonstrated that loss of WRN function causes the production of telomeric circles in fibroblasts expressing telomerase. To determine the mechanism whereby the WRN complex regulates telomere homeostasis, we propose to mechanistically characterize the functional interplay between WRN, Ku and telomere-specific factors found at chromosome ends and define the process activated by loss of WRN function responsible for the formation of telomeric circles. To accomplish these objectives we propose the following three aims. In Aim 1, we will characterize the role of WRN and its interacting partner Ku70/80 in the regulation of telomeric termini in human cells. In Aim 2, we will dissect the molecular mechanisms leading to the formation of extrachromosomal t- circles resulting from loss of WRN function. In Aim 3, we will characterize the biochemical properties of WRN in the context of model telomeric substrates in vitro. The studies proposed in this application will elucidate the role of WRN at telomeres and provide valuable information for understanding how loss of WRN function promotes genome instability, premature aging and the early onset of diseases such as cancer and cardiovascular disease. It is recognized that the incidence of these diseases increases progressively with age. Therefore, the changes at the molecular, cellular and physiologic levels that occur during aging profoundly influence the development and progression of these diseases. Functional analysis of WRN will provide a clearer understanding of these changes and will help in the development of therapeutic agents aimed at preventing the early onset of age-associated diseases.